As will be familiar to the skilled person, wells are used to extract resources such as oil, gas, bitumen, etc. from buried reservoirs.
In general, and as the skilled person is aware, gathering information about the physical environment within and surrounding a well is useful both in terms of understanding what level of reserves are present and ensuring that the reserves are recovered in an efficient, effective and economical manner. Therefore, geophysical surveying, including seismic surveying, is usually carried out at various times throughout well development and use. While traditionally such surveying was carried out using geophones or hydrophones (use of which is generally restricted to observation wells), fibre optic sensors are becoming a well-established technology for a range of applications. This includes the use of downhole fibres, which can be placed while the well is being constructed and remain in place throughout the lifecycle of the well, and are interrogated with optical radiation when information is required. Such fibres are robust and versatile, and are capable of operating in environments which would damage or destroy geophones and hydrophones.
The fibres may contain sensor portions (for example, Fibre Bragg Gratings (FBGs) can be used to form interferometers used to monitor strain in the fibre portion between the two FBGs) or may operate as distributed fibre optic sensors, such as Distributed Acoustic Sensor (DAS) fibres, in which the intrinsic scattering sites within the fibre provide a backscatter return signal.
In DAS sensing, a single length of (typically single mode) fibre which can be unmodified, in the sense of being free of any mirrors, reflectors, gratings, or (absent any external stimulus) any change of optical properties along its length can be used. One example of a DAS fibre is described in GB2442745, the content of which is hereby incorporated by reference. Such a sensor may be seen as a fully distributed or intrinsic sensor as it uses the intrinsic scattering processes inherent in an optical fibre and thus distributes the sensing function throughout the whole of the optical fibre. Further examples are provided by WO2012/137021 and WO2012/137022. The content of these three applications is incorporated herein to the fullest extent possible.
WO2012/123760 is an application which describes the use of fibre optics in seismic surveying, and is incorporated herein to the fullest extent possible.
The skilled person will be aware of multiple bore systems. There are various circumstances in which two or more well bores are provided in relative proximity in a well system. For example, in order to extract oil efficiently from certain oil fields, in particular those which contain viscous oil or bitumen deposits, steam is sometimes used usually with the primary purpose of increasing the temperature of the deposit (thereby lowering its viscosity), in large part by transferring heat as the steam condenses. For example, in Steam Assisted Gravity Draining (SAGD), when a reservoir containing a viscous resource deposit has been identified and geology allows, two bores are drilled, both with horizontal sections in the reservoir, an upper shaft running above a lower shaft. To allow thick, tar-like resources to flow, steam is injected through the upper shaft (and also, in some wells, initially through the lower shaft) causing the resource to heat up, liquefy and drain down into the area of the lower shaft, from which it is removed.
Other related techniques are ‘steam flooding’ (also known as ‘continuous steam injection’), in which steam is introduced into the reservoir though (usually) several injection well shafts, lowing the viscosity, and also, as the steam condenses to water, driving the oil towards a production well shaft.
However, as will be appreciated by the skilled person, there are various other circumstances in which multiple bores are advantageous, be that vertical, horizontal or so called ‘directional’ wellbores. For example, such systems can provide effective access to a hydrocarbon resource whilst concentrating the above-ground infrastructure to a relatively localised area. In some examples, observation wells are drilled in relatively close proximity to other wellbores and which are used to conduct (amongst other things) seismic surveying.
There remains a need to accurately and conveniently provide downhole surveying, both in single and multiple bore wells.